Electronic ignition system for an internal combustion engine

ABSTRACT

An electronic ignition system provides precisely timed spark plug ignition in the combustion chambers of an internal combustion engine and provides an optimal supply of ignition energy for the required operating mode by including a programmable transistor ignition system (&#34;PTI&#34;) controllable according to engine operating parameters which can execute a joint program for all spark plugs. The PTI is connected in series with high-voltage capacitor ignition devices associated with the individual spark plugs and supplies ignition energy to them repeatedly during each ignition process.

BACKGROUND OF THE INVENTION

This invention relates to electronic ignition systems for internalcombustion engines.

Published European Application No. 0 071 910 A2 discloses an electronicignition system having individual spark plug ignition devices as well asan additional ignition device common to all spark plugs. In that system,transistor ignition devices with current regulation are used as theindividual spark plug ignition devices and the common ignition devicecontains a regulated direct-current converter connected, by way ofdiodes, to the secondary windings of the ignition coils of thoseignition devices. Both types of ignition devices may be controlled orregulated as a function of parameters of the internal combustion engine,such as rpm, load and knocking.

A disadvantage of this prior art ignition system is that essentiallyonly a single ignition pulse is supplied by the direct-current converterfor each ignition process. As a result, difficulties may occur inignition with a lean mixture, especially in operating modes of theinternal combustion engine which are of interest in view of modernreduced emission objectives.

In this respect, more favorable behavior is provided by a programmabletransistor ignition system, hereinafter called a "PTI", such as isdisclosed in German Offenlegungsschrift No. 23 40 865. That PTI containsan electronic switch connecting a direct-current voltage source into anoutput transformer and having a switching frequency which is a multipleof the firing frequency of each spark plug. Like the conventionalignition device, the PTI is controllable according to operating andenvironmental parameters. A disadvantage of this known PTI is therequirement for a mechanical distributor, which is known to berelatively susceptible to trouble. In addition, the PTI does not supplyvery precisely timed ignition sparks.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a fullyelectronic ignition system for an internal combustion engine whichovercomes the above-mentioned disadvantages of the prior art.

Another object of the invention is to provide a fully electronicignition system which ensures satisfactory precisely timed ignition,even when the internal combustion engine is operated with a leanfuel-air mixture, and which does not require a mechanical distributor.

These and other objects of the invention are attained by providing anignition system in which each spark plug is supplied from a separatehigh-voltage condenser ignition device, hereinafter called an "HVCI",having an output transformer and a plurality of spark plugs arecontrolled by a common PTI having an electronic switch operating at afrequency which is a multiple of the firing frequency of each spark plugand which controls the operation of the HVCI for each of the spark plugsas a function of operating and environmental parameters.

The use of a PTI as an ignition control device which supplies a seriesof spark impulses for each ignition process produces a relatively strongionization of the combustion chamber gases in the vicinity of the sparkplug, which results in greater assurance of complete ignition than doesthe use of a single direct-current converter. Because each spark plug isconnected to a separate HVCI and an electronic distributor is used as inthe transistorized coil ignition devices disclosed in theabove-mentioned EP No. 0 071 910 A2, a mechanical distributor is notrequired.

Although the ignition energy supplied by each HVCI is limited, the HVCIshave a high timing accuracy. Consequently, in the ignition systemaccording to the invention, the actual ignition energy is provided tothe HVCIs by the PTI with relatively limited timing accuracy, while theHVCIs provide precisely timed high voltages to the individual ignitioncoils.

An additional advantage of the invention results from the fact that theHVCIs can be standard components since all of the features of theignition system required by a particular engine are determined by thePTI.

Another advantage of the invention is that HVCI devices are conventionalcomponents as described in the extensive patent literature in thisfield, such as European Published Application No. 0 001 354 A1.

As can be seen from the following description of particular examples,the ignition system according to the invention permits the use of asimple diagnostic device that supplies informative data. A particularadvantage of the invention in this connection is that a centraldiagnostic device may be used which is common to all combustionchambers. In engines for motor vehicles, this device may be built intothe vehicle so that, if necessary, the driver may be given informationconcerning the status of the ignition system, including the spark plugs.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the invention will be apparent from areading of the following description in conjunction with theaccompanying drawings in which:

FIG. 1 is a schematic block wiring diagram illustrating the arrangementof a conventional PTI;

FIGS. 2-5 are schematic diagrams illustrating the arrangement of severalrepresentative embodiments of an ignition system according to theinvention;

FIG. 6 is a schematic representation of the magnetic circuits in theembodiment of FIG. 5;

FIGS. 7-11 are schematic diagrams showing additional representativeembodiments of the invention;

FIG. 12 is a schematic diagram showing the arrangement of one form ofdiagnostic arrangement for use in the invention;

FIGS. 13 and 14 are graphical illustrations showing typical curvesproduced by the diagnostic arrangement of FIG. 12;

FIGS. 15 and 16 and FIGS. 17 and 18 are graphical illustrations showingvoltage and current curves, respectively, which are produced by thediagnostic arrangement for various insulation failures in the ignitionsystem;

FIGS. 19 and 20 are graphical illustrations showing voltage and currentcurves as a function of the electrode gap of the spark plug;

FIGS. 21 and 22 are graphical illustrations showing voltage and currentcurves during interruptions of the combustion in the ignition process;

FIG. 23 is a schematic diagram showing another example of a diagnosticarrangement;

FIGS. 24 and 25 are graphical illustrations showing voltage and currentcurves in the diagnosis of engine knocking;

FIGS. 26 and 27 are schematic diagrams showing two further embodimentsof a diagnostic arrangement; and

FIGS. 28 and 29 are graphical illustrations showing voltage and currentcurves indicating the failure of an HVCI.

DESCRIPTION OF PREFERRED EMBODIMENTS

As can be understood from the foregoing summary, the PTI forms anessential component of an ignition system arranged according to theinvention. The basic design of a PTI ignition system, as disclosed inGerman Offenlegungsschrift No. 23 40 865, will therefore be describedhereinafter with reference to FIG. 1.

The electronic components of the ignition system shown in FIG. 1 areconnected through a positive line 1 and a negative line 2 to adirect-current voltage source 3 by way of an ignition switch 4 disposedbetween the positive line 1 and the positive terminal of the voltagesource 3. The direct-current voltage source 3 may have a voltage of, forexample, 12 V, as is customary in motor vehicles.

A direct-current voltage converter 6 is connected through a filter 5 tothe positive and negative lines 1 and 2. The filter 5 is a conventionallow-pass filter consisting of inductances and condensers which isolatesthe switching frequency of the direct-current converter 6 from thedirect-current voltage source 3 to prevent disturbance of thedirect-current voltage supply.

The direct-current voltage converter 6 may be a conventional push-pullconverter or a single-ended converter arranged to convert the voltage ofthe direct-current voltage source 3 to a direct-current voltage of, forexample, 50-100 V, preferably 70 V. The output of the direct-currentvoltage converter 6 is connected to the input of a conventional currentregulator 7 in which the actual value and the desired value of thecurrent (i.e., the ignition energy) are compared. The desired valuesetting is provided by three control elements 8, 9 and 10, which may bepotentiometers, for example. The control element 8 is actuated as afunction of the setting of the output control element of the engine, thecontrol element 9 is actuated as a function of the ignition time point,and the control element 10 is actuated as a function of the fuel-airratio.

A voltage regulator 11 connected to the current regulator 7 to maintainthe output voltage constant has its output side connected to the primarywinding of an ignition transformer 12.

One terminal of the primary winding of the ignition transformer 12 isconnectable through an electronic switch 13 to the negative referencepoints of the current regulator 7 and to the voltage regulator 11. Theswitching frequency of the on or off status of the switch 13 isdetermined by a pulse generator 14 in conjunction with two timingelements 15 and 16 and an ignition timer 17. Switching of the switch 13thus induces an oscillating voltage in the primary winding of theignition transformer 12, which is transformed into a high voltage in thesecondary winding.

The switch 13 consists essentially of transistors arranged, for example,in a Darlington circuit, along with resistors for adjustment of theoperating point of a switching transistor which is represented in thedrawing as a switch 13'. The pulse generator 14 connected to the switch13 operates in a conventional manner, for example, as an astablemultivibrator, and a potentiometer 18 varies the frequency of the pulsegenerator 14 to optimize the switching frequency to the transmissioncharacteristics of the ignition transformer 12. The pulse generator 14is switched on and off for preselectable intervals of time by the timingelement 15, which may be designed as a conventional monostablemulti-vibrator. The switch-on interval is variable within wide limitsand the desired value setting is effected by four control elements 19,20, 21, and 22, which may, for example, be potentiometers. The controlelement 19 is actuated as a function of the rpm, the control element 20is actuated as a function of the setting of the output control element,the control element 21 is actuated as a function of the ignition timepoint and the control element 22 is actuated as a function of thefuel-air ratio.

The timing element 15 is switched on by the timing element 16, which maybe designed as a conventional monostable multivibrator, and the timedelay which is initiated by the timing element 16 for the start ofignition energy generation is variable within wide limits. The desiredvalue is determined by three control elements 23, 24 and 25, which maybe potentiometers, again as a function of rpm, setting of the outputcontrol element and fuel-air ratio. The timing element 16 is switched onby the ignition timer 17 which serves to initiate the ignition processby opening a breaker contact.

The alternating high voltage produced in the secondary winding of theignition transformer 12 has a frequency which is determined by the pulsegenerator 14 and an effective duration which is determined by the timingelement 15, where triggering by the ignition timer 17 is effected inconjunction with the timing element 16. This voltage is supplied througha rectifier 26 and a distributor 27 to a series of spark plugs in acylinder head 28.

The advantage of a PTI of this type is that, by supplying a series ofignition sparks as a function of parameter values for each mixtureignition, it provides greater assurance of effective ignition due tostronger ionization.

Accordingly, such a conventional PTI is provided as an important part ofthe ignition system according to the invention to supply ignition energyto each of the spark plugs during its corresponding combustion orburning times. During the combustion time, a succession of individualpulses is generated, each of which leads to an ignition spark. Asdescribed above, the current amplitude of each pulse and the pulsefrequency may be freely varied as a function of engine parameters. Theignition energy of each ignition process consists of the currentamplitude of each individual pulse, its time duration and the number ofpulses within the duration of combustion, which is also freely variable.

The present invention eliminates the effect of certain timinginaccuracies in the operation of the PTI, as well as avoiding thenecessity for a mechanical distributor in the PTI by providing anindividual HVCI for each spark plug to produce an ignition discharge foran exact time period at the spark plug. The basic design of the HVCIs isexplained hereinafter with reference to the typical example of theinvention illustrated in FIG. 2.

In this example, as in the other examples, a four-cylinder internalcombustion engine with four spark plugs 30, 31, 32 and 33, one for eachcombustion chamber, is assumed. Common to all four spark plugs 30-33 isa PTI connected between two terminals 34 and 35, which has theconventional design described above with reference to FIG. 1 except thatit does not include the distributor 27. The PTI is therefore indicatedonly by the reference numeral 36 in FIG. 2 and in the remaining figures.

As shown in FIG. 2, a diode 38 is connected in series with the outputtransformer 37 of the PTI 36 and a condenser 39 is connected in parallelwith this series connection.

Four HVCIs 44, 45, 46 and 47 with corresponding ignition coils 40, 41,42 and 43 are assigned to the spark plugs 30, 31, 32 and 33,respectively. These HVCIs have the same arrangement, which is shown indetail only for the HVCI 44 assigned to the spark plug 30, and they areidentified by the same reference numerals in the other drawings sinceall of the HVCIs may have the same structure. As shown in FIG. 2, theseHVCIs are connected to the primary windings of the ignition coils 40-43.

Referring to the arrangement of the HVCI 44, a direct-current converter49 is connected to the positive terminal 48 of the vehicle battery and acondenser 50 is connected between the converter and the ignition coil40. A switching transistor 51, connected between the condenser 50 andground, is rendered conductive by a triggering device 147 as a functionof output signals from control unit 145, which is acted on by signalsfrom a Hall device 146 of a distributor. The condenser 50 is thusdischarged through the primary winding of the ignition coil 40 whenignition is to be effected by the spark plug 30 in the associatedcombustion chamber.

To make certain that the ignition energy is not transmitted to the otherspark plugs 31, 32 and 33, which are also connected through a junction52 to the PTI 36, in other words, that ignition does not occur in the"wrong" combustion chambers, care must be taken that the voltage at thejunction 52 between the PTI 36 and the HVCIs 40-43 does not exceed agiven value, for example, 1.5 kV. This is accomplished by appropriateselection of the condenser 39 and the diode 38 based on the inductanceof the secondary winding of the output transformer 37 of the PTI 36.

Between the diode 38 and the junction 52, there is a diagnostic unit Dwhich will be described hereinafter with reference to other figures. Atthis point, it should only be noted that the invention provides forconvenient installation of a diagnostic unit, as shown in FIG. 2, whichpreferably is incorporated in the vehicle containing the internalcombustion engine.

In the description of the following examples, the reference numeralsused for certain parts in FIG. 2 are used for corresponding parts in theother embodiments illustrated in subsequent figures.

Referring to the typical embodiment shown in FIG. 3, this embodimentdiffers from that of FIG. 2 by including four individual spark pluginductances 53, 54, 55 and 56, inserted between the junction 52 and theHVCIs 44-47, respectively, to each of which is assigned a capacitor 57,58, 59 and 60. These capacitors may constitute the winding capacitancesof the inductances 53-56. The inductances 53-56, in cooperation with thecondenser 39, prolong the duration of the corresponding HVCI ignitionspark and, for small ignition currents, intensify the ionizing effect ofthe sparks produced by the HVCIs. Accordingly, their significance is inproviding a "transition" between the HVCI and the PTI in cases in whichthe ignition energy produced by the PTI 36 does not cause ionization inthe corresponding cylinder charge.

In the further embodiment of the invention shown in FIG. 4, individualspark plug secondary windings 61, 62, 63 and 64 of the outputtransformer 37 of the PTI 36 are connected in series with correspondingsecondary windings of the ignition coils 40-43. The magnetic coupling ofthe secondary windings 61-64 with the primary winding 65 of thetransformer 37 is represented in the drawing by a bar 66. Fourcondensers 67, 68, 69 and 70, which correspond in function and size tothe single condenser 39 of the previous embodiments, are connected inparallel to the secondary windings 61-64. In this embodiment, the diode38 is located in the ground connection of the ignition system. Ifdesired, individual diodes could instead be connected to each of thesecondary windings 61-64. This embodiment provides the advantage ofsubstantial decoupling of the ignition processes for the individualspark plugs 30-33 from each other.

The same advantage is provided by the circuit illustrated in FIG. 5, inwhich the locations of the HVCIs 44-47 and the PTI 36 are reversed withrespect to FIG. 4.

FIG. 6 shows the magnetic coupling of the secondary windings 61-64 ofthe output transformer 37 of the PTI with the associated primary winding65 through individual cores 66a, 66b, 66c and 66d.

As can immediately be seen from FIGS. 7 and 8, which are the same exceptthat the sequences of individual circuit components are transposed, theembodiments of FIGS. 3 and 4 or of FIGS. 3 and 5 may alternatively becombined into a single circuit. In the embodiments shown in FIGS. 5 and8, care must be taken with regard to insulation since the PTI 36 is at ahigher voltage than in the other embodiments.

The further embodiments of ignition systems shown in FIGS. 9, 10 and 11are similar to those of FIGS. 3, 7 and 8, and the corresponding partsare identified with identical reference numerals. In these embodiments,the inductances 53, 54, 55 and 56 constitute the secondary windings oftransformers 71, 72, 73 and 74, which are driven by external excitationand serve to control the transition between the modes of operation ofthe HVCIs and the PTI. If desired, additional ignition devices may beconnected to the input terminals of the transformers 71-74. Preferably,however, the PTI 36 provides the input to those transformers since italready has a transformer.

So far, the diagnostic unit D has only been mentioned and has not beendescribed in detail. As may be seen in FIGS. 2-11, the ignition systemaccording to the invention provides especially convenient arrangementsfor insertion of a diagnostic unit for detecting failures which mayoccur in any part of the ignition system. A number of appropriateembodiments of the diagnostic unit are described hereinafter withreference to the drawings.

In the embodiment shown in FIG. 12, the diagnostic unit D has twoterminals 80 and 81. The terminal 80 corresponds, for example, to thejunction 52 in the embodiment of FIG. 2. The diagnostic unit D in FIG.12 contains a voltage sensor formed by an LED 83 connected in serieswith a resistor 82 and a current sensor formed by an LED 84 in the linebetween the terminals 80 and 81. Conventional photosensitive units 85and 86, with light-transmitting elements 87 and 88 and photoresponsiveelements 89 and 90, are arranged to generate voltage- andcurrent-indicating signals. Accordingly, an electrical signalrepresenting the voltage curve with respect to time (t), as shown inFIG. 13, may be produced at the terminals 91 and 92 and an electricalsignal representing the current flow with respect to time (t), as shownin FIG. 14, may be produced at the terminals 93 and 94. In FIGS. 13 and14, the PTI is switched on by the control system at a time A and, aftera time period t₁, it charges a condenser, for example, the condenser 39in FIG. 2, up to an amplitude of, for example, 1.5 kV, which is reachedat the time A' in FIG. 13. The corresponding HVCI is then ignited at thetime B and during the time period t₂ the burning condition of the arcbetween the electrodes of the spark plug is represented by the line F inFIG. 13 and the line G in FIG. 14.

A variety of failure conditions will now be described with reference toFIGS. 15-20.

A crack in the insulating ceramic of the spark plug, i.e., an insulationfailure, becomes apparent, as shown in FIGS. 15 and 16, because ofvoltage disruptions or current peaks during the time period t₁. Thisfailure may therefore most easily be detected by integration of thevoltage and current curves during the time period t₁. This integratedvalue i up to the time A' of the voltage curve or up to the time E ofthe current curve is stored in the diagnostic unit in a conventionalmanner and is evaluated by comparison with the integrated values whichare obtained in the normal condition of the ignition system.

Integration of current and voltage during the time period t₁ also showsthe effect of a leak due to soot or to a coating of moisture in coldstart. FIG. 17 shows the voltage curve and FIG. 18 shows the currentcurve with respect to time (t), three different degrees of fouling a, band c being assumed in the voltage curve. The integrated value i isdrawn on a different scale, as it is in FIGS. 15 and 16. On the otherhand, the electrode gaps of the spark plugs under consideration aremonitored by their effect on the combustion voltage in the time periodt₂. To eliminate fluctuations due to the respective operating states, itis advantageous to average the integrated values i shown in FIGS. 19 and20.

The fluctuation of current with time is a measure of the flow pattern ofthe fuel-air mixture in the combustion chamber. FIG. 21 shows theintegrated value i of the voltage in the range t₂ with the flow patternin the combustion chamber varying with time. The flow of the mixturecauses the arc between the electrodes to drift without interrupting theburning thereof. Only when flows are strong can interruptions of thecombustion occur. FIG. 22 shows the corresponding current curve, fromwhich it can be determined whether the burning was interrupted.

The diagnostic units shown in FIGS. 23 and 26 are designed to detectknocking. Considering first the arrangement shown in FIG. 23, atransformer 102, connected between two terminals 100 and 101, issupplied from a power stage of the PTI through two terminals 103 and 104and a diode 105. The operation of this diagnostic unit is based on thefact that the motion of the cloud of electrons during the combustionprocess is modulated by any knocking that may occur. In such knocking,the modulation frequency is in the range from 5-15 kHz. After completionof the ignition process, i.e., within a time period t₃, a positiveaccelerating voltage with a frequency of, for example, 75 kHz, isapplied through the transformer 102 to the central electrodes of thespark plugs. The modulation frequency is thereby scanned, as it were,and the voltage curve illustrated in FIG. 24 can be measured at anoutput terminal 106 or between two further output terminals 107 and 108.The current curve shown in FIG. 25 may be used to control thecorresponding current amplitude.

The diagnostic unit D of FIG. 26 constitutes a variation of that of FIG.23, and the same reference numerals are used for corresponding parts. Inthis case, however, a direct voltage rather than an alternating voltageis applied as the accelerating voltage to the terminals 103 and 104 forthe transformer 102 with a rectifier 109 and a smoothing condenser 110being connected across a resistor 111 and hence across the spark plugelectrodes. A compromise must be made in this circuit with reference tothe selection of the resistor 111. If the resistance is too high, itreduces the magnitude of the current and if the resistance is too lowthe power loss is too great.

Alternatively, the circuits of FIGS. 12 and 23 and of FIGS. 12 and 26may be combined, if desired. The first combination is shown in FIG. 27,which consequently uses the reference numerals of FIGS. 12 and 23.

These diagnostic units also permit the detection and localization ofadditional failures. Thus, FIG. 28 shows the current curve and FIG. 29shows the voltage curve in the event of failure of an HVCI, in each casewith respect to time (t). When this happens, the PTI runs in thedirection of its no-load value until the voltage on the electrodes ofthe spark plug in a combustion chamber during the exhaustphase producesa discharge. At that time, burning of the arc takes place only during asmall part of the intended duration.

The embodiment of FIG. 27 includes two additional connectable anddisconnectable integrators 112 and 113 for voltage and current,respectively, having output terminals 114 and 115. These terminalsproduce control or regulating signals which are at least intermittentlyconnected to one or more of the control elements 8, 9, 10 and 18-25 inthe PTI shown in FIG. 1. Thus, the ignition frequency of the PTI and/orthe ignition energy and/or the duration of combustion and/or the timepoint of connection may be made dependent upon the status of theignition system and the corresponding combustion chamber. This assuresthat any ignition difficulties are not only detected but are eliminatedor compensated for.

The invention therefore provides an ignition system which, whileavoiding mechanically moving parts, supplies ignition energy which isdetermined by the operating parameters of the internal combustion engineto corresponding spark plugs at precisely predetermined times andprovides an advantageous opportunity for use of a diagnostic systembuilt into the vehicle.

Although the invention has been described herein with reference tospecific embodiments, many modifications and variations therein willreadily occur to those skilled in the art. Accordingly, all suchvariations and modifications are included within the intended scope ofthe invention.

We claim:
 1. An electronic ignition system for an internal combustionengine having a plurality of spark plugs comprising a plurality ofhigh-voltage condenser ignition devices (HCVIs) having outputtransformers and switches controlled by signals of an ignitiondistributor, each HVCI assigned to a corresponding spark plug to providea precisely timed spark energy source, at least one common ignitiondevice comprising a programmable transistor ignition system (PTI)connected in series with the individual HVCIs and common to at least onegroup of spark plugs as in ignition energy generator influenceable byengine parameters, the PTI including a direct-current voltage source, anoutput transformer and an electronic switch having a switching frequencywhich is a multiple of the firing frequency of each spark plug and whichis effective to provide pulsewise cut-in of the direct-current voltagesource to its output transformer during predetermined intervals of timethrough timing elements which are controllable as a function ofoperating and environmental parameters.
 2. An ignition system accordingto claim 1 wherein the secondary circuit of the PTI output transformerincludes a capacitor selected in accordance with the inductances of thesecondary windings of the transformers of the HVCIs so that the voltagegenerated by the PTI in those secondary windings is limited to preventundesired ignition processes.
 3. An ignition system according to claim 1including a plurality of inductances connected in series with the PTIand the corresponding HVCIs to prolong the ignition signals provided bythe HVCIs for small ignition currents.
 4. An ignition system accordingto claim 3 wherein each of the inductances is a component of anexternally excited transformer for controlling transition behaviorbetween the PTI and the HVCIs.
 5. An ignition system according to claim1 wherein the PTI output transformer has a separate secondary windingconnected to each of the HVCIs.
 6. An ignition system according to claim5 wherein separate iron cores having a common primary winding areprovided for each of the separate secondary windings.
 7. An ignitionsystem according to claim 5 wherein the spark plugs are connected to thePTI output transformer.
 8. An ignition system according to claim 1including diode means for isolating the PTI from the HVCIs.
 9. Anignition system according to claim 1 wherein the spark plugs areconnected to HVCI output transformers.
 10. An ignition system accordingto claim 1 including diagnostic means having voltage and/or currentsensor means connected in series with the PTI.
 11. An ignition systemaccording to claim 10 wherein the diagnostic means is connected betweenthe PTI and ground.
 12. An ignition system according to claim 10 whereinthe diagnostic means is connected between the PTI and the HVCIs.
 13. Anignition system according to claim 10 wherein the diagnostic meansinclude light-transmitting means for transmitting signals from thesensor means.
 14. An ignition system according to claim 10 wherein thediagnostic means includes at least one integrator for integratingvoltage and/or current signals with respect to time.
 15. An ignitionsystem according to claim 14 wherein the integrator is effective duringthe time between the starting time of the PTI and the ignition time ofeach of the HVCIs to detect insulation defects.
 16. An ignition systemaccording to claim 14 wherein the integrator is effective during thetime an ignition spark burning voltage is applied to a spark plug todetect defective ignition conditions.
 17. An ignition system accordingto claim 10 wherein the diagnostic means is arranged to supply anaccelerating voltage to the spark plugs after cut-off of thecorresponding HVCIs and includes an evaluating circuit for detectingoscillations in the kilo cycle frequency area to provide indications ofknocking.
 18. An ignition system according to claim 17 wherein theaccelerating voltage is a direct-current voltage.
 19. An ignition systemaccording to claim 17 wherein the accelerating voltage is an alternatingvoltage with a frequency which is a multiple of the frequency of anyoscillations to be expected.
 20. An ignition system according to claim10 wherein output signals from the diagnostic means are utilized ascontrol signals for the PTI.